This invention relates generally to the production of electromagnetic radiation in the region of 0.5-100 nanometers by the irradiation of target materials with high energy laser beams and more particularly to production of the target materials.
The generation and use of extreme ultraviolet (EUV) radiation has wide applicability in the fields of materials science, microlithography and microscopy. Two frequently used sources of such radiation are a laser-produced plasma and synchrotron radiation. With appropriate modification laser plasma sources are as bright as their more expensive synchrotron counterparts, but are better suited to a small laboratory or commercial environment.
While laser plasmas are efficient EUV sources, they are plagued by the problem of unacceptably high production of damaging atomic and particulate debris which arises as a consequence of the method of producing the plasma. The ejection of such debris limits the applicability of plasma sources. Laser plasma sources typically employ planar solid metal targets that are irradiated with pulsed lasers at incident intensities in the range of 10.sup.11 -10.sup.13 watts/cm.sup.2. Under these conditions, the laser creates a 10-100 eV plasma above the target surface. The hot expanding plasma may exert pressures greater than 100 kilobar back onto the target, leading to melting and/or fragmentation of the target and the ejection of fast, hot particulate matter. These ejecta range in dimensions from atomic size to greater than 10 micrometers in diameter. The atomic size ejecta are effectively stopped from damaging nearby optics by the use of low pressure gases. In contrast, the large particulate ejecta, which can travel at speeds up to 10.sup.5 cm/sec are very difficult to block or divert. This debris adheres to and/or erodes nearby solid surfaces and is thus extremely deleterious to optics and other instrumentation placed near the plasma. The magnitude of this problem is illustrated by the fact that as little as 150 .ANG. of a typical target material such as gold deposited on a multilayer-coated mirror will reduce its reflectance to 13% of the original value. Because a "zero-debris" laser plasma source of EUV and soft x-ray radiation would expand greatly the applicability of these sources to commercial and scientific applications which cannot tolerate debris-induced damage, there has been a concerted effort to develop debris mitigation methods as well as novel target geometries to reduce or eliminate this problem.
One method of mitigating this problem is to place optics at certain angles with respect to the laser plasma target surfaces thereby reducing the exposure to debris. Another strategy for reduction of energetic target debris is the use of "mass-limited" targets. U.S. Pat. Nos. 4,872,189, 4,837,793 and 5,151,928 disclose target geometries in which tapes or ceramic membranes support thin metal films thus limiting the volume of target material exposed to the laser beam thereby reducing, but not eliminating, the total amount of debris ejected. However, these tape systems are difficult to construct and prone to breakage necessitating opening the vacuum system and interupting operation of the system. U.S. Pat. No. 4,866,517 discloses the use of cryogenic solid targets to avoid the formation of condensable solid debris. These targets are made from frozen inert or rare gases such as krypton, or xenon. Because of the cost involved in the use of these gases an elaborate containment and recovery system is necessary in order to keep the costs below $10.sup.-6 per shot. U.S. Pat. No. 4,723,262 discloses the use of discontinuous droplets of a metal having a melting point below 100.degree. C., such as mercury, gallium, indium, cesium or potassium, as the target material. In order to minimize the production of debris the droplets are sized to correspond to the size of the laser plasma beam. This requires elaborate control equipment as well as means for heating those metals whose melting points are greater than 25.degree. C. The production of some target debris is inevitable. Furthermore, because of the low vapor pressure of these materials at room temperature, they will tend to condense on optical components causing losses in optical properties. The use of fast mechanical shutters and magnetic shutters, as described in U.S. Pat. Nos. 4,866,517 and 4,969,169, has also been proposed to block or divert debris, however, they are known to be only marginally effective because of the speeds at which the ejecta travel. None of these schemes has demonstrated the ability to simultaneously achieve significant debris reduction while maintaining adequate source brightness or collection solid angle.
Another scheme has been used to generate laser plasma targets; free-jet expansion of gases. Hot, dense plasmas have been produced by high power laser interaction with small gas clouds, or clusters, formed by pulsed injection of gas through a nozzle (free-jet expansion) into a vacuum chamber. Results of an experimental investigation into x-ray generation from Nd3+:YAG laser-irradiated free-jet expansion gas targets indicated that this technique could be used to produce intense soft x-ray emission (Laser plasma x-ray source with a gas puff target, H, Fiedorowicz, A. Bartnik, P. Parys, and Z. Patron, Inst. Phys. Conf. Ser. No 130, Chpt. 7, p 515, IOP Publishing Ltd., 1993, and SPIE 1994). In the work of McPherson, et al. (Applied Phys. B 57, p 337, 1993, Phys. Rev. Lett. 72, p 1810, 1994), clusters were used to study the mechanisms of x-ray production where extremely high laser intensity, 5.times.10.sup.16 -8.times.10.sup.18 watts/cm.sup.2, short-pulse (300 femtoseconds) laser sources were used to produce cluster-enhanced x-ray emission. It has been demonstrated that clusters do provide benefits in the direct multiphoton induced emission of specific x-ray transitions. However, there has been no work reported in which it has been demonstrated that the use of molecular clusters can result in minimization of plasma debris while at the same time serve as efficient targets for the generation of EUV or soft x-rays where traditional laser plasma heating schemes are used.